Metropolitan L.A. Under a Slow Squeeze

Downtown and West Los Angeles are moving toward the San Gabriel Mountains and the metropolitan area in between is being and will be squeezed slowly over the next several thousand years, according to researchers using precise satellite surveying techniques at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA.

The measurements suggest that new mountains may be forming to the south of the high San Gabriel Mountains.

The results come from the Southern California Integrated Global Positioning System (GPS) Network, an array of 60 current and 250 planned GPS receivers that continuously measures the constant, yet tiny, movements of earthquake faults throughout Southern California.

"We've known for some time that the area between the coastline and the Mojave Desert is being squeezed together by the constant movement of Earth's crust," said Dr. Donald Argus, a geophysicist at JPL. "This new research helps pinpoint the area that's being squeezed. Specifically, downtown and West L.A. appear to be moving toward the San Gabriel Mountains at about half a centimeter (one-fifth of an inch) per year."

Argus is presenting his finding Oct. 29 at the annual meeting of the Geological Society of America in Toronto, Canada.

"While this research does not mean that an earthquake in Los Angeles is imminent, one possible conclusion is that the earthquakes that occur in Los Angeles might be concentrated in the northern part of the basin," Argus said.

The GPS surveying system uses radio signals transmitted from a constellation of 24 Earth-orbiting satellites that are jointly operated by the U.S. Departments of Defense and Transportation. Equipment on the ground receives signals from several satellites at a time, allowing scientists to pinpoint the position of a receiver to better than 1 centimeter (0.4 inch).

"The regional project is designed for exactly this kind of study. Our goal is to observe and monitor the slow, small motion, called strain, of the ground in greater Los Angeles," said JPL's Dr. Frank Webb, chair of the Southern California network. "This research helps us learn where earthquakes are more likely to happen, and helps with estimating the regional earthquake hazard in Southern California. It enables other agencies to make priorities about earthquake mitigation activities, including emergency preparedness and retrofit strategies."

There are now about 60 GPS receivers on the ground around Southern California with two new sites being added every week. The earthquake network began in 1990 with only four GPS receivers as a prototype project funded by NASA. It detected very small motions of Earth's crust in Southern California associated with other California earthquakes in June 1992 in the town of Landers and in January 1994 in Northridge.

The Southern California network includes a number of institutions using GPS for earthquake research. The consortium is coordinated by the Southern California Earthquake Center, a National Science Foundation science and technology center headquartered at the University of Southern California. The array is operated by JPL, USC, the U.S. Geological Survey and the Institute of Geophysics and Planetary Physics at the University of California at San Diego's Scripps Institution of Oceanography.

The JPL research is part of NASA's Earth Sciences Enterprise that seeks to understand the total Earth system and the effects of natural and human-induced changes on the global environment. JPL is a division of the California Institute of Technology.